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I am in online business since 3 years and I know very well about xml sitemap and html sitemaps. First of all I want to say “bhadriram” has explained it in proper way.

I want to focus on xml sitemap because it matters a lot if you want to be cached fast and quickly by Google Search engine.

As you know google crawlers converts a webpage into xml after that it starts reading or crawling therefore, if your website already have an xml sitemap then googlebot will easily read content of your website.

The conclusion is that xml sitemap is necessary for each and every website.

XML site map is very useful tool to tell the search engines about the pages of your website. It is automated and does not need to update again and again when you add a new page to your website. It is convenient when your website has hundreds of pages. In case of HTML site map you need manually update each changes about of your website.

Both the site map is important for the crawler as well as for visitors. HTML site map works as a index of a website from which visitors can know the inter related page and where they are.. And XML site map acts as a path finder for Google crawler to index your page and you can also give the time span when to crawl your website.
Robots.txt can be applied in the XML site map location.